In some manufacturing operations there is a need to maintain two surfaces substantially parallel to each other within a very high degree of tolerance (i.e., a few arcseconds). An example of such a process is the mounting of a small-sized semiconductor chip, such as a semiconductor laser chip, to a mounting surface, i.e., a substrate or the like. Typically the semiconductor chip, which may be on the order of 10 mils .times.10 mils, is mounted such that each of a plurality of conductive members (i.e., solder bumps or the like) on the undersurface of the chip is to be brought into registration with a corresponding bonding pad on the substrate. Thereafter, a reflow operation is conducted to bond the chip to the substrate.
The planarity (i.e., parallelism) of the semiconductor chip with the substrate is critical in order to assure that each mounting pad on the chip makes aligned contact with a corresponding bonding pad on the mounting surface. Even a small degree of non-planarity between the chip and the substrate can adversely affect the reliability of the resultant electrical connection. Often, the semiconductor laser chip is relatively expensive so achieving a high degree of parallelism during assembly is critical if scrap is to be minimized and costs are to be controlled.
The planarity of the chip with the substrate is also critical for another reason. Often the chip itself is very delicate, especially when the chip is fabricated from III-V or similar compounds which are brittle and tend to have poor mechanical integrity, when compared to the mounting substrate which is often made from silicon or diamond. When the chip is relatively brittle as compared to the mounting substrate, a lack of planarity between them can cause the chip to fracture or otherwise become damaged when mounting is attempted.
The planarity of the chip with the substrate is generally controlled by the mechanism used to place the chip on the substrate. Typically, a vacuum pickup tool, having a flat pickup surface, is manipulated by a robotic arm or semi-automatic device to pick up the chip and place it on the substrate which is securely held on a flat, rigid mounting base. Usually, the orientation of the pickup tool can be adjusted to achieve planarity (within a prescribed tolerance) of the chip with the substrate. In the past, a laser interferometer has been required to measure the planarity of the pickup tool and mounting bed. Such an interferometer is costly and is difficult to operate. In addition, certain safety regulations may have to be employed when using such laser interferometers.
Thus, there is a need for a less costly and less complex method of measuring the planarity of two surfaces.